The answer is ... it depends! What is your objective for using a social media site? Who are your customers and which social media sites do they use?
There is no "best" site - each site has different features and benefits which work differently according to the sector you are in, what kind of customers you have and your specific objectives for using social media.
Furthermore it depends upon your employees. If someone tells you Facebook is the best, yet your staff hate it, no amount of encouragement from you will get them to use it to benefit your business.
So you need to find out more about your customers and how they use social media, more about the preferences of your staff and focus in on some clear objectives for using social media.
Simply being told a particular site is the best will not take into account the specific and individual circumstances at your company I'm afraid.